Supporters of Michael Sata, the leader of the opposition Patriotoc Front (PF), gathered outside the main counting centre in the capital on Sunday after results indicated that their man had slipped from an early lead on Friday to third place.
With results confirmed from two-thirds of constituencies on Sunday, the country's president, Levy Mwanawasa, looked set to win a second and final five-year term.
Mwanawasa, head of the Movement for Multi-Party Democracy (MMD), had a third of a million more votes than Sata after results from 120 of the 150 districts.
Although the tally does include results from some constituencies in urban areas, Sata said the commission was sitting on the outcome from others.
"I have won these elections. Let them announce the elections from our Lusaka, Copperbelt, Northern and Luapula provinces where we have won overwhelmingly," Sata told reporters.
Tensions were high on the
streets of Lusaka
"We demand that results from our stronghold areas be released forthwith. What criteria are they using in releasing the results?"
With only 20 per cent of polling stations to declare Sata was lagging in third place behind Hakainde Hichilema, a wealthy businessman who is relatively new to politics.
Protesters were bundled into police trucks as tensions rose with many shops deciding to shut and residents remaining indoors as PF supporters forced some roads to close and harrassed motorists.
Despite government assurances that Thursday's vote had been fair, Sata, says he will lodge a protest over alleged missing ballots and has said there will be "severe consequences" if his objection is ignored.
A paramilitary unit patrolled the business district and took up positions at the privately owned Post newspaper, where Sata supporters had protested on Saturday.
State radio said the paper, which had predicted a landslide win for Mwanawasa, had been placed under police protection.
About four million Zambians are registered to vote.
Mwanawasa, 58, based his campaign on his economic record, which included winning billions of dollars in debt relief and boosting economic growth above five per cent.
Sata has warned of
Sata, 69, a populist known to his supporters as "King Cobra", has appealed to the poor with wealth redistribution and tax-cutting policy proposals.
Even though Zambia is one of the world's biggest copper producers, more than two-thirds of the population live on less than $1 a day.
The electoral commission was initially praised by international observers for an efficient and transparent election, but has now come under criticism for its slowness in releasing results.